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Full text of "Mathematical And Physical Papers - Iii"

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280          ON  THE  CHANGE  OK   UKKKAXGIIUMTY  OK   LIGHT.
anticipated.    The principal fixed lines of the violet, and of the chemical parts of the spectrum beyond, were seen with beautiful distinctness as dark  planes interrupting an otherwise perfectly continuous mass of blue light.    To see any particular fixed line with  most distinctness, it was of course  necessary to hold the eye in the corresponding plane, when the dark plane was foreshortened into a dark line.    From the red end of the spectrum, as  far  as  the   line   G, or   thereabouts, the   light  passed  freely through the fluid, or at least was only reflected here and there from motes held in  mechanical  suspension.    About  G the dispersion just commenced to be sensible, and there were traces of that line seen as a dark plane interrupting a mass of continuous but excessively faint  light.    For some distance further on the dispersed light remained so faint that it might have been passed over if not specially looked for.    It was about half-way between G and II, or a little before, that it first became so strong as to arrest  attention, and   a  little   further  on   it   became very conspicuous, the tint meanwhile* changing to a pale sky-blue.    The light was very copious about the two broad bands of the group II and for some distance from   //  towards  ({.    Some of the fixed lines less   refrangible   than  //  were  very  plain,  and  beyond H a  good  number   were   visible,  which   will   presently  be   further described.     The   whole,   system   of   fixed   lines   thus   visible  as interruptions in the dispersed light had a. resolvable appearance; but with a very narrow slit and a lens of long focus at the prisms the light would have been too faint for convenient observation.
The dispersed light about (7, and for some distance further on, was so very faint that I might have overlooked it. had it not arrested my attention when observing by the fourth method; indeed, I have sometimes specially looked for it in the third arrangement without having been able (<> see it. Practically speaking, the dispersion might be said to commence, about halfway between G and //.
17. On refracting the whole system sideways through a prism of moderate angle held in front of the eye, the fixed lines became confused, and the finer ones disappeared. The edges of the broad bands H were tinged with prismatic colours, like the edges of two slips of black velvet placed on a sheet of pale blue paper, and viewed through a prism. This experiment exhibits the